Wireless personal area networks based on OFDM and UWB technologies like the MB-OFDM standard will directly interfere with narrowband interferers that are close to such wide band devices. At present, no specific interference mitigation techniques are implemented in the UWB standard based on OFDM.
Orthogonal frequency-Division Multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels (sub-carriers) at different frequencies. To avoid in-band spectral interference, it is proposed to reach a given attenuation level within a given transmission band of the interfering signal.
For example, one method includes modulating (or zeroing) the transmitted sub-carriers located within the given transmission band to try to cancel the signal energy found in this given band. However, because of side lobe amplitudes of the remaining transmitted sub-carriers, it is generally difficult to reach relatively high attenuation levels within a given band of the transmitted OFDM symbol. These side lobes are produced by the sin(x)/x shape spectrum of each transmitted sub-carrier.
In particular, in the coexistence perspective of MB-OFDM systems with other wireless systems, and particularly with WiMax, notches of as much as −30 dB with respect to the OFDM sub-carriers transmission power are requested.
Another type of method permitting one to obtain relatively high attenuation levels within a given band of a transmitted signal is disclosed in “Active Interference Cancellation Technique for MB-OFDM Cognitive Radio,” Hirohisa Yamaguchi, 34th European Microwave Conference—Amsterdam 2004.
More precisely, according to this method, the band between sub-carriers s1 and s2 of an OFDM symbol is notched by: 1) zeroing every sub-carrier between s1 and s2, 2) modulating sub-carriers s1 and s2 using coefficients computed as a function of the OFDM symbol transmitted data, with the target of minimizing the energy in the band s1-s2.
However, such a method is relatively complicated to implement.